Pointfree Calculator

29072008

Intro

In this semester had to do this project with my friend, João Moura, under supervision of professor Alcino Cunha.

The project was to make a program that make automatic the process of proving an equality of functions written in the pointfree style. Not completely automatic, but so iterative. The program load a file with rules and the user inserts an equality of functions in pointfree and can surf the words to find a sub-expression which want to run a particular rule. From the side are shown all the rules that may apply to selected sub-expression.

What is pointfree?

Pointfree is a style of programming that helps us to compose functions. Even more, it helps us later to prove equality between functions.

How can someone prove equality between functions? We can do so, because we have rules. Let’s see an analogy with algebraic calculus.

We have proved that .

Pointfree calculus as is done now, on paper, is boring and repetitive. A proof is this image of someone trying to make a proof.

Abstract representation

If swap is a functions for exchange the elements of a pair:

swap (a,b) = (b,a) --in pointwise

swap = split snd fst --in pointfree

And we want to prove that: . This is clearly obvious, but I will use this example just to show you the representation that we use to see these proofs.

So, here we have the representation of :

If you make a inorder passage it make sense.

As you can see the operator have arity n. Because we want that this operator became flat, we never want to use the law:.

Function f is variable, and swap is constant, we know their definition.

Imagine now that we load the following file of rules:

Path

In order to know that sub-expression are to select a term, we have the Path which behaves as follows:

If we select 4 in the following expression , the Path becomes: .
If we select 2, the Path becomes: .

the list in front of the Path is to select a range of elements in an operator with arity n. Thus, if we want select in , the Path becomes: .

Proof of

This process is pretty always the same, but I suggest that you follow with some attention.

In this first tree I will show the Path already selected.

Here I will show the rule that will apply in our expression, and the instantiation of that rule to our selected sub-expression.

Selected function:

Path:

Rule:

Instantiation of the rule:

So, the resulting tree will be:

Resulting function: .

Proof of

Selected function:

Path:

Rule:

Instantiation of the rule:

So, the resulting tree will be:

Resulting function: .

Proof of

Selected function:

Path:

Rule:

Instantiation of the rule:

So, the resulting tree will be:

Resulting function: .

Proof of

Selected function:

Path:

Rule:

Instantiation of the rule:

So, the resulting tree will be:

Resulting function: .

Proof of

Selected function:

Path:

Rule:

Instantiation of the rule:

So, the resulting tree will be:

Resulting function: .

Proof of

Selected function:

Path:

Rule:

Instantiation of the rule:

So, the resulting tree will be:

Resulting function: .

Proof of

Selected function:

Path:

Rule:

Instantiation of the rule:

So, the resulting tree will be:

Resulting function: .

Proof of

Selected function:

Path:

Rule:

Instantiation of the rule:

So, the resulting tree will be:

Resulting function: .

From the rule of equality that we have is true and therefore it is true

Presentation

Time to show the software interface.

The program’s interface is divided into 3 parts:

Input

Here the user can insert something that want to prove, for example:
and so on…

When you hit enter, you start having fun calculating :)

Rules that may apply

Here you can navigate in the rules pressing PgUp or PgDn, and enter to aply the selected rule to the selected sub-expression.

Proof state

Here you can browse the expression imagining it as the trees that showed earlier.

up/down – to navigate into the levels

left/right – no navigate into childs

backspace – to make undo in the proof

We can also save and load proofs into XML, and also save proofs to PDF.

Conclusion

Me and João still improving the pointfree calculator, and in the next year it will be used in one course here in Universidade do Minho.
The next stage we will implement type inference in the pointfree calculator, to make it powerfull. A lot of things have to be done, and in the next month we will start doing that, including make the first public release of the software.

If you understand Portuguese and want to see the presentation that we give in Department of Informatic in University of Minho:

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5 responses

29072008

Chaddaï Fouché(21:53:29) :

Very nifty, will you make this software freely available ? I know that I myself wished for an UI quite like that (I imagined something more graphical, but your representation may be easier to work with in big trees).
It was in a wider context than pointfree computation though : I was using Coq and I noted that some very simple demonstrations became uselessly complex because of the lack of such a tool.

@Chaddaï Fouché
Thank you for your comment. Not only do we distribute the Pointfree calculator free as we will also put it under an Open Source license. But that will only happen after program do type inference, because only then the calculator is indeed a powerful tool.
Today we already can do proofs for recursive functions, {cata, ana, hylo}morphisms, however it is still necessary to specify all of these morphisms rules for each data type. We want to be as general as possible. With the inference of types we will also increase the range of rules, because there are rules that work only on certain types of data.
Later I will announce when we start to distribute the software.